This proposal outlines a strategy for defining key components of a novel type of dynamic stem cell niche in the Drosophila ovary and for elucidating the mechanism of competition for niche occupancy. Accomplishing the aims in this proposal will be an important milestone toward the long-term goal of building a detailed model of epithelial stem cell regulation in order to develop regenerative medicine-based therapies and understand how the regulatory process fails in disease states such as cancer. The follicle stem cells (FSCs) that produce follicular epithelia have many features in common with mammalian epithelial progenitors and will likely be an excellent model for epithelial stem cell biology. Like several other types of epithelial stem cells in Drosophila and mammals, FSCs are stably maintained in the tissue but do not appear to be supported by a fixed, non- dividing niche cell population. Instead, putative niche cells divide during adulthood, and the position of the niche changes as surrounding germ cell cysts move through oogenesis. In addition, FSC daughters regularly migrate between niches and compete for niche occupancy in a process that may have broad relevance both to normal epithelial homeostasis and the early stages of cancer. However, the identity of the putative niche cells and the mechanisms that mediate stem cell maintenance and niche competition are unknown. Based on published studies and significant preliminary data, the central hypothesis of the proposal is that differences in cell adhesion and cell polarity cause FSC and daughter cell fates to diverge. Specific aims include: (1) Identifying the dynamic FSC niche cells, determining their lineage, and measuring their rate of turn over; (2) Elucidating the role of cell adhesion in FSC maintenance and FSC niche competition; and (3) Elucidating the role of cell polarity in the specification of FSC versus daughter cell fate. To achieve the first aim, cell-type-specific markers, lineage analysis, and cell cycle markers will be used to identify putative niche cells and measure their rate of turnover, and results will be confirmed with live imaging. To achieve the second and third aims, immunofluorescence will be used to identify cell adhesion and cell polarity proteins that are expressed in the FSC niche, and mosaic analysis will be used to determine their function in FSC maintenance and competition. This project is significant because it will establish a new model for the study of epithelial stem cell niches. It is an innovative departure from previous studies that focuses on building a comprehensive model of the FSC niche and on understanding the novel concepts of the dynamic niche, niche competition and stem cell-specific polarity. Ultimately, it will provide a foundation for understanding conserved mechanisms of in vivo epithelial stem cell regulation.